Hello!
It would be really nice to have a "key lookup" feature in evolution that
lets you download PGP
keys from a keyserver if a message is signed but the public key of the
sender is not in the key ring.
Any chance that this will be implemented in a future version?

GPG should do that automatically if you assign a keyserver in the config
file for gpg.
I don't think that feature exists in current versions of PGP.Since
things have been migrating to gpg anyways (since NAI is pretty much
abandoning PGP), I am not certain if the effort would be worth it.
(Unless you want to support PGP 2.x keys and that is a different can of
writhing vermicelli.)